The present invention relates to improvements in the field of waste reclamation. More particularly, the invention is directed to the recovery of commercially valuable products from automobile shredder residue.
The automobile industry, in its effort to meet stringent automobile fuel economy standards by manufacturing light weight automobiles, has selected plastics to replace heavier metals as the construction material for numerous components. Generally, the life of an automobile is 9 to 10 years, at which time it is dismantled and/or shredded into ferrous and non-ferrous metals and shredder residue.
Automobile shredder residue is the remaining material after the shredding and demetallizing process. It is composed of rubber, glass, fabric, plastics, insulation, fibrous items, stones and dirt. Generally, the inorganic matter content of automobile shredder residue ranges from about 35 to about 65 weight %, whereas its organic matter content is within the range of about 65 to about 35 weight %, on a dry basis. The plastics, fabrics and fibers are petroleum derivatives and are a part of the millions of tons of petroleum products that are landfilled annually.
The disposal of automobile shredder residues in landfills has become a major environmental problem. Each year, as many as 10 to 11 million scrapped cars are shredded in North America. Since automobile shredder residue contains more and more mixed plastics materials, decomposition of landfill shredder residue may take millions of years to return to organic matter.
It has already been proposed in Published International Patent Application No. WO 88/08020 to treat by pyrolysis wastes of mixed polymeric materials derived from the demolition and grinding of cars. The pyrolysis is carried out at temperatures ranging from 300.degree. to 800.degree. C. under an atmosphere of inert gas, such as nitrogen. The material used as feedstock comprises rubber, plastics, wood, metals and inert substances. The heat treatment of such a material at a pressure of 760 mm Hg or above favors the repolymerization and recondensation of organic macromolecules derived from the thermal decomposition of the rubber and plastics over the surfaces of the char-forming material and metals, thereby coating same with repolymerized and recondensed secondary products which are then charred and eventually cokefied as they are subjected to high temperatures. As a result, the metals recovered are coated with layers of coke which strongly adheres to the metal surface, thus rendering the metals unsuitable for direct recycling. The adherence of the coke of the metal surface is so strong that in order to eliminate the coke, the coated metals must be heated at temperatures above 1000.degree. C., which renders the recovery of metals commercially unattractive.
In addition, the wood which is present in the feedstock undergoes at high temperatures decarboxylation and dehydration, producing carbon dioxide and water vapors which are oxidizing agents that can interact with the hot metals to produce metal oxides. The presence of these oxidizing agents together with the high temperature and atmospheric pressure conditions enhance undesirable chemical reactions of the metals, which alter the metal surface.